srm: Improve error message on file_busy and duplication_errors

I think these messages are more to the point. The duplication error
message is taken from the SRM spec.

Target: trunk
Request: 2.7
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6223/